Ruts filled with water sparkle in flashes of midday sun, and behind us a rainbow arches towards the north coast. Belts of dark conifers and the flat greyness of Crowdy reservoir mark this waterlogged land on the northern edge of Bodmin Moor. In the distance, beyond the potholed runways and ruins of a former RAF Coastal Command base, a modern factory that makes cheese stands out. Ahead, in the opposite direction, some three miles to the south, Brown Willy (420m) sticks up above the intervening moorland.

Near the start of this approach to the highest hill in Cornwall, the isolated outcrop of Lanlavery Rock used to be a popular place for picnics and gatherings of local church groups and schools. Today, the undulating expanse of splashy Davidstow Moor is deserted, swept by squalls and overlooked by a band of cattle looming on a nearby horizon. Tawny grasses are woven with the pale Cladonia portentosa lichen – shaped like tiny trees in miniature forests, fluffed up and almost visibly expanding in the damp atmosphere. Beside a fenced plantation of dripping conifers with mossy trunks, sheltering bullocks have left a morass of deep mud. Marshland appears red and brown while the drier slopes of turf and partially submerged granite are scattered with sheep, all startlingly white on the drab hillsides.

Up on Showery Tor, the precariously balanced boulders glisten as sun emerges through storm clouds. The nearby craggy outline of Rough Tor is silhouetted against the brightening sky, and just visible, on the coast to the north-west, are Stepper Point and Trevose Head. On the distant southern part of the moor, Kilmar, Sharp Tor and Stowes Hill remain gloomy under louring clouds. Across the De Lank valley, the top of Brown Willy is temporarily enveloped in mist but, as we descend towards the river crossing, sunlight glints on wind turbines situated upstream and beyond the watershed. The two-bladed versions ranged across Kittows Moor appear to move jerkily compared with the more stately three-bladed versions at Bowithick.

Across the river, and away from the ancient settlements and enclosures of Roughtor Moor, the eroded path runs with water. It leads uphill, past tumbled walls, narrow gateways between granite posts and tussocks of wind-blown grasses which gleam in the low sun. In front of us, four walkers cross the boggy rushes and ling at the foot of the rocky summit, in advance of the next bout of rain. No time for us to linger on top, but we see sun reflected on Dozmary Pool and Colliford reservoir as dark clouds encroach on the Cornish Alps or china clay country. The Tamar valley remains bathed in sunlight for another few minutes as stinging rain follows us downhill.

Sign up for the Guardian Today

Our editors' picks for the day's top news and commentary delivered to your inbox each morning.